i havent been here in quite some time, looking for some design advice.

i am building a nifty landscape lighting supply (four 12vdc output channels, mosfet controlled outputs, with PWM driver in the mix). each channel will be rated/regulated 12vdc @ 5amp.


my questions:

1) to get good regulation the DC(in) to reg should be ~15v possibly more. thus a std 12vac(out) xfrmer wont work, need something like a 15vac or higher xfrmer? i can get some decent inexpensive 24vac/5amp xfrmers. would 24vac xfrmer be ok for most linear regs?
2) is it more efficient to build full wave rectifier with two diodes and CT xfrmer, or non CT xfrmer and four diodes?
3) its cheaper to get four 5amp xfrmers than one 20amp item. should i build four discrete power supplies in one case, or can i parallel the xfrmers (phased together) and just build one large power supply? i would use a hefty bridge rectifier but i would then use individual regs for each channel.

advice?

 

A 12 volt transformer:
(12 times sqrt 2) minus 1.4V for the diodes minus 3V for the regulator chip = 12.57 volts.
That leaves 0.57 volts of p-p ripple on the filter capacitor.
A single winding with 4 diodes is the best use of the transformer because all the turns are working on every half cycle. You can only get 62% of the rated current out of that configuration. 5amps/.62 = 8.06 amp rating needed.
You can parallel the windings if the transformers are a matched set and you pay attention to get the phasing right.
My preference is to build 4 separate supplies so the whole setup doesn't go down for a single failure, and it's easier to get rid of the waste heat.
Using a low dropout regulator chip will allow the filter capacitors to be smaller.
Building only one supply is cheaper, I think.
You are free to use something different.

One thought is a pile of (4) 60 watt switching supplies which might be bought cheaply and quickly.

 

A 12VAC transformer will output a peak voltage of 12V x 1.4 =16.8V

Subtract two diode drop of 0.7V will give 16.8V - 1.4V = 15.4VDC which should be just about right for you.

Your questions are answered here:

Hammond Transformer Design Guide

Unregulated Power Supply Design

 

this would be quick & easy
http://invertersupply.com/index.php?main_page=product_info&products_id=2782

 

An ATX psu will give 22Amps @ 12V easy.....cheaper

 

I like the number of neophytes that come here wanting to get hands on and build stuff they could buy for just a small sum of money. It gives me hope for our engineering future.

 

this would be quick & easy
http://invertersupply.com/index.php?main_page=product_info&products_id=2782

I guess these would snap into a panel of some kind. Can you please point me to info on how this would be setup? I'd love to get rid of the rats nest wiring that supplies my 12v lights currently. Thanks

 

i would put the rail into a metal enclosure which is being mounted outside (metal chassis with clear plastic front will suffice given my mounting location) . since this is for landscape lighting each unit line in would be controlled via a 4 channel timing unit that controls a relay to power each PS. i will build a mosfet stage to handle the 12vdc channels so i can connect some already made PWM controllers.

post #6. whaaaaat, its why people come here, to learn and build even if its cheaper to buy joe-doodle's item off of ebay, etc. in this case though, the PS i listed will save me $$ and time.